This invention is concerned with the recovery of underground hydrocarbons by the injection of a gaseous recovery fluid. More particularly, the invention pertains to the use of at least two wells, one of which is employed as a continuous injection well, and a second well which is operated in a cyclic injection and production manner.
Several methods of enhanced oil recovery have involved the injection into an underground formation of a gaseous solvent such as carbon dioxide. The injected fluid may be injected at conditions so as to make the fluid miscible or conditionally miscible with the underground hydrocarbons. Numerous tests have been performed in the field with the injection of carbon dioxide through one or more injection wells to drive underground hydrocarbons to one or more producing wells.
Cyclic oil recovery processes wherein injection and production of fluids takes place through the same well are also known in the art. One of the earliest disclosures of a cyclic oil recovery process was in U.S. Pat. No. 3,480,081 wherein the flooding medium was water, brine or steam. The success of steam cyclic recovery processes inevitably lead to the injection of carbon dioxide in a cyclic or push/pull process wherein carbon dioxide was injected into an individual well, allowed to soak, and then produced. U.S. Pat. No. 4,390,068 discloses such a carbon dioxide cyclic process. Cyclic carbon dioxide recovery has now become a commonplace event in the oil field.
Attempts to recover heavy oils and hydrocarbons from tar sands have lead to a number of processes involving the injection of various solvents and hot fluids in "pressurization and drawdown methods." These are similar to cyclic carbon dioxide methods in that various solvents and fluids are injected into the formation through a well to increase formation pressure. The fluids may or may not be allowed to soak in the formation prior to producing the injected fluids along with hydrocarbons through the same well. U.S. Pat. No. 4,324,291 is one example of these processes.